This invention relates to the field of corrosion-resistant alloys and more particularly to low strategic metal content, nonmagnetic, workable alloys resistant to both oxidizing and reducing sulfuric acid solutions over a wide range of acid concentrations and temperatures.
It is now well recognized that alloys of the iron-nickel-chromium-molybdenum-copper types, which offer good resistance to both oxidizing and reducing solutions of sulfuric acid, are also quite resistant to rusting and to a wide variety of corrosive media and conditions. Such resistance has, in fact, been demonstrated for alloys of practically no iron content up to alloys of considerable iron content. For example, an alloy known commerically as Illium R, consisting nominally of 68% nickel, 21% chromium, 5% molybdenum, 3% copper, 1% iron and small amounts of other elements has such corrosion resistant properties. Another alloy commerically known as Carpenter 20, consisting of nominally 29% nickel, 20% chromium, 2.5% molybdenum, 3.5% copper, 42% iron, and small amounts of other elements has been very widely and successfully applied in all sorts of corrosive conditions and exhibits relatively good resistance to the various sulfuric acid solutions noted above. There are a number of other alloys ranging in iron content between these two with similar corrosion resistance properties.
I have noted these same characteristics in a number of alloys of my own invention which are of the same type, namely, iron-nickel-chromium-molybdenum-copper base with additions of certain other elements. It is known that such alloys generally tend to be substantially insensitive, to sea water, to salt air, and to industrial atmospheres and a variety of other corrosive atmospheres.
While various stainless steels resist general attack in certain ranges of sulfuric acid concentration and other corrosive media, a majority of them are susceptible to a pitting type attack, crevice corrosion, and stress corrosion cracking failures in chloride media such as sea water or the aqueous fluids often encountered in solar heating systems. Such fluids typically contain 180-1200 ppm Cl.sup.- and 5-400 ppm Cu.sup.++. It is now well known that additions of the order 0.5 to 3% by weight molybdenum can alter the properites of standard stainless steels in a manner that remarkably increases their resistance to pitting, crevice corrosion, and stress corrosion cracking failures. This has been demonstrated repeatedly for many stainless steels. The most successful stainless steel at present for solar heating applications contains nominally 18% chromium, 2% molybdenum and small amounts of other elements with the remainder consisting of iron. But this alloy is of ferritic crystal structure and therefore strongly magnetic.
For various services, particularly certain military and naval applications, it is highly desirable that construction materials be substantially non-magnetic. Thus, for example, to provide immunity to magnetic detection, it is of critical importance to construct missile-firing submarines from non-magnetic alloys. Fixed site missiles can, of course, be located, and the detection of surface vessel movements is fairly readily achieved by satellite reconnaissance. The chief advantage of the missile-firing submarine is that it is movable and capable of evading detection. Available locating techniques for submarines include sonar, thermal detection systems, sea-bed sensors and magnetic anomaly detection.
Thermal systems are useful only in shallow water. Sea-bed sensors may be evaded. Sonar has already been developed to near the limit of practical possibility. Present sonar transmitters and receivers are powerful enough already to be limited by turbulence and temperature effects in the ocean. However, unless an ordinary steel submarine is very deep and stationary, the presence of so large a mass of moving magnetic metal causes a disturbance in the earth's magnetic field which can be detected by sensitive apparatus. As this equipment is refined, magnetic steel submarines will become more and more vulnerable to attack from airborne missiles or from enemy killer submarines.
Substantially non-magnetic reactor materials such as aluminum, zirconium, titanium and others are well-known, but the greatest metallic mass of the hull, decks, bulkheads and structural parts are generally not fabricated at present from such materials. For example, titanium alloys have remarkable resistance to sea water, but would be virtually impossible to use in hull construction at anything but enormous cost, if at all. Integrity of welds in so vast a structure as an atomic submarine is not feasible at present.
Austenitic alloys, that is alloys of facecentered-cubic crystal structure, may and generally do posses relatively low magnetic permeabilities of the order of 1.2 gauss/oersted or less, as compared to maximum permeabilities for various iron materials ranging from about 100 to 15,000. Ferritic alloys generally exhibit relatively high permeabilities. While alloys of magnetic permeabilities of the order of 1.003 to 1.007 at 200 oersteds can be considered substantially nonmagnetic, permeabilities of abut 1.01 characterize very weakly magnetic materials and are probably still very much below the tolerance levels for use in such applications as the construction of naval vessels such as mine-sweepers or atomic missile-firing submarines.
Lange, Howells, and Bukowski at the U.S. Naval Research Laboratory, as far back as 1958, reported development of various alloys that finally led to employment of alloy steels of about 0.4% carbon, 18% manganese, 4% chromium, and 0.1% nitrogen in actual submarine construction. However, such alloys have proven unsatisfactory due to cracking in service.
A long list of precipitation-hardening stainless steels is now available. These include alloys of good mechanical and fabricating properties but all exhibit substantial magnetism.
Most standard grades of wrought stainless steels develop permeabilities up to 5, 10, even 20 gauss/oersted, during rolling or cold working as the result of structural instability leading to the formation of ferrite, the amounts of which depend upon the grade of stainless and the degree of cold working. Even where not forged or wrought, standard grades of cast stainless steels may display considerable magnetism due to the presence of amounts of ferrite or carbides and nitrides or other compounds in their structure.
Post and Eberly reported in the transactions of the American Society for Metals of 1947 mathematical relationships between the elements in austenitic stainless steels such that they remained structurally stable and substantially nonmagnetic for alloys containing iron, nickel, chromium, molybdenum, carbon and manganese. Schaeffler in 1948 extended the knowledge to include silicon, niobium, and nitrogen, and subsequent workers have recognized that up to a few percent of copper is mildly austenitizing. Franks, Binder and Thompson extended the limits to include up to about 22% manganese in reports in various publications including the 1954 Transactions of the ASM and U.S. Pat. No. 2,225,440.
Thyssen Rohrenwerke Aktien-gesellschaft revealed in British Pat. No. 1,062,658, alloys containing iron, nickel, chromium, manganese, molybdenum, carbon, silicon, nitrogen and niobium and mathematical relationships between these elements to maintain nonmagnetisability.
In 1924, Gustav Tammann of Germany suggested his rule of eights describing the effects upon corrosion resistance of additions of atomic fractions of 1/8, 2/8, 4/8, or 7/8 of a noble or resistant element alloyed with a baser or less noble or non-resistant element in various media. Under this concept, one expects the very poor resistance in a given corrosive medium for lesser or non-resistant elements, but there would be stepwide reductions in corrosive attack when the more noble or more resistant element additions reach amounts corresponding to specific atomic percentages. Gradual increases between these amounts have virtually no effect, but sudden drops in corrosion rates are expected at several if not all of these concentrations.
In practice there are very few alloys of iron and nickel base which contain over about 38% chromium by weight, for higher chromium levels tend to result in severe embrittlement problems. Also, there is generally little if any improvement in resistance to corrosion in most media for additions of chromium above this level. Indeed, there are few conditions in which chromium levels over about 22 to 24% are warranted, though levels up to 27 or 28% by weight are regularly specified because effective chromium is somewhat depleted in the formation of carbides, nitrides, or other compounds and to provide some range of tolerances for production purposes.
At the other end of the chromium range, the atomic fractions of 1/16 or 1/8 provide only enough passivating protection to be useful in very special or relatively weak corrosive media. About 4 to 6% by weight chromium was once used in cutlery grades to resist the mild action of fruit and other food juices. However, even the 11 to 12% chromium range is not efficacious in most severely corrosive substances. The major drop in corrosive rate in most highly corrosive substances takes place at chromium levels corresponding to the 3/16 or 1/4 atomic fraction levels of chromium.
In U.S. Pat. Nos. 3,759,704 and 3,893,851 I disclosed alloys of excellent general corrosion resistance but of particularly good resistance to wide ranges of sulfuric acid concentrations and temperatures, with chromium levels of 33 to 42% by weight, or at approximately the 3/8 atomic fraction. Johnson U.S. Pat. No. 3,758,296 also teaches sulfuric acid resistance alloys of this general level of chromium. Later I disclosed alloys for the same service but of the lower chromium levels of 23.3 to 30% by weight in Culling U.S. Pat. No. 3,947,266 and 4,135,919. The latter was actually superior to the former despite its reduced strategic metal content and resultant permissible increase in iron content.
While there are ore deposits for molybdenum in the U.S., this metal is in such demand relative to the supply that it is almost as strategically critical at times as are nickel and chromium, both of which depend almost entirely upon imports. U.S. copper deposits are virtually exhausted so that even this element is often in short supply. Niobium is also an imported metal and metallurgically more desirable than titanium and tantalum. Alloys of U.S. Pat. No. 4,135,919 were actually equal or superior to U.S. Pat. No. 3,947,266 despite effective reductions of about 5% in nickel content, 4% in chromium content, 2% in molybdenum content and smaller amounts of copper and niobium.
There has remained the need to further reduce the proportions of strategic elements in alloys of this type without significant loss in workability and weldability while maintaining excellent corrosion resistance. The list of U.S. and foreign patents disclosing alloys of about 20% or more chromium by weight to handle various sulfuric acid solutions is very long, yet tests disclose many of them to be either quite inferior to my prior inventions, or to contain extremely high proportions of strategic elements, or to suffer severe mechanical limitations such as extreme brittleness, or to have all three railings. Alloys of lower chromium contents have suffered even more drastically in these deficiencies.